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Hi guys, does anybody know why 2-stroke ignition advance curves are opposite to 4-stroke.


4-stroke, more rpm, more advance(you fire the spark plug earlier):



2-stroke, more rpm, less advance(you fire the sparkplug later):





Thanks!

Bye bye.

Easy. As you probably know, in four stroke engines, you advance the timing (that is, it happens before the TDC) to impulse the piston at the right moment. As fuel takes time to burn, you start the ignition earlier when the piston is moving faster to kick it at the same moment.

Now, when you retard the ignition in two stroke engines, the ignition occurs closer to the moment when the exhaust "valve" opens. So, you are getting hotter gases into the pipe. Speed of sound is faster in hotter gases. Gases also have more energy, so they leave the pipe faster, giving you more intake from scavenging. This is the first reason.

The second one is a little more complicated. The time to peak pressure (after you ignite the combustible) is smaller with larger RPM because of more turbulence in the mixture. At larger RPM the increase in turbulence is larger, so you have to retard the ignition to keep peak pressure at the same timing.

For example (I really think that you need numbers to get any idea):

C2/C1 = sqrt(T2/T1)

That is, the speed of sound (C) is proportional to the square root of temperature.

So, if you have gases at 400 degrees you have a speed of sound of 520 m/s, give or take.

Let's say that you have a two stroke pipe tuned to 10000 RPM. Now, we have some clever way (retarding ignition anyone?) to add 100 degrees to temperature of exhaust gas. This means that by the empirical relation for CD (crank displacement), which is:

CD = (0.012 * tuned-length * RPM) / speed-of-sound.

The pipe will actually be tuned to 11900 rpm with 100 degrees more (that is, you keep constant the relationship between RPM and speed of sound in the previous equation).

Would you like to displace your power band by 1900 rpm? I'd say you would.

Awesome as usual, Ciro. Big Thanks.

The question that follows is obvious. Do you have any recommended readings on tuning of 2-stroke engines? I might tune 2-stroke engines in the not so near future(a year from now, maybe). I already have some plans for an inertial dyno and was looking for pressure sensors which could be put inside a 2-stroke exhaust.